Weighbridges



March 16, 1965 D. w. CLAMP WEIGHBRIDGES Filed April 1. 1963 UnitedStates Patent Ofiice 3,173,597 Patented Mar. 16, 1965 3,173,507WEIGHBRIDGES David Warwick Lloyd Clamp, Knowle, England, assignor toJoseph Lucas (Industries) Limited, Birmingham, England Filed Apr. 1,1963, Ser. No. 269,335 Claims priority, application Great Britain, Apr.17, 1%2, 14,779/62 6 Claims. (Cl. 177-211) This invention relates toweighbridges of the kind including a plurality of load cells to whichthe load to be measured is applied.

It has been found that when using such weighbridges the readings of theweighbridge may vary with the position of the load on the Weighbridge.This effect results from employing load cells having unequal outputimpedances. If a load is eccentrically positioned on the \veighbridge itmay effectively short-circuit one or more of the load cells. This wouldnot matter if the load cells were of equal output impedance, but wherethey are not the above-mentioned inaccuracy arises.

The object of this invention is to minimise this difiiculty.

The invention resides in a method of manufacturing a weighbridge of thekind specified, including connecting the load cells in parallel to acommon output cable through resistors respectively the resistances ofwhich are chosen so that the output impedances of the load cells areequal.

The invention further resides in a weighbridge of the kind specified inwhich the load cells are connected in parallel to a common output cablethrough resistors respectively, the resistances of the resistors beingso chosen that the output impedances of the load cells are equal.

As a further optional feature, the load cells are connested in parallel.to a common input cable through resistors respectively whereby thesensitivities of the load cells can be varied.

The present invention is one of a series of live applications numbered269,335 to 269,339 and all relating to weighbridges. The accompanyingdrawing is a diagram illustrating one example of the present inventionas applied to a wei-glrbridge incorporating features which arespecifically claimed in the other four applications in the series.

Referring to the drawings, the apparatus includes a weighing station 6at which the actual weighing operation is performed, and a remoterecording station '7. The wcig'hbridge at the weighing station includesa plate (not shown) for receiving vehicles or other goods to be weighed,the plate being supported at its four corners by four bridge-type loadcells Each load cell 8 incorporates four strain gauges (illustrated asresistors 9) connected in a bridge network, deformation of two of thegauges under load serving to provide an output from the bridge which isdependent on the load, and the other two gauges providing temperaturecompensation in known manner.

The A6. input supply to the four load .cells is supplied from a source11 at the recording station through a cable 12 extending from therecording station to the Weighing station, the cable incorporating atransformer 13 intermediate its ends. The transformer 13 is positionedat the weighing station as close as possible to the load cells 8, sothat the length of cable between the transformer 13 and the load cellsis kept to a minimum. The effect of this is to keep the voltage dropalong the length of cable feeding the load cells to a minimum. It willbe appreciated that where the cable extends between the recording andweighing stations, its length may be substantial so that the effect ofthe voltage drop along the cable may result in inaccurate results.However, by the arrangement described above, the length of the cable between the transformer and the load cells is small, so that ernors due tothe length thereof are minimised. Moreover, the importance of any errorsdue to the relatively long cable between the supply and the transformeris reduced in the ratio of the transformer.

The inputs to the four load cells are fed through four resistors 14respectively, the resistances of these resistors being adjusted on thesite to determine the sensitivity of the respective load cells 8.

The output from the four load cells 8 are fed through four resistors 15respectively, these resistors being pre-set on installation of theweighbridge to ensure that the output impedances of the four load cellsare equal. This point is of considerable importance if, as is usuallythe case, it is possible to weigh a load distributed unevenly withrespect to the load cells. In such a case, one of the load cells may ineffect be short-circuited. This will not matter if the output impedances of all the load cells are equal, but if they are unequal,different readings will be obtained depending upon which load cell isshortcircuited.

The output cable 16 from the load cells to the recording station is not,as is common practice, combined in a four-cable with the input cable 12.The output cable is separated from the input cable by a sufiicientdistance to obviate transformer effects on the output current as aresult of current flowing in the input cable. It is found that the useof a four-core cable (as is common in electrical weighbridges) resultsin inaccurate readings as a result of such effects.

The output cable is connected to a measuring circuit at the recordingstation, the measuring circuit including first, second and thirdsecondary transformer windings 17, 18, 19 and first, second and thirdresistors 20, 21, 22 connected across these windings. The secondarywindings may be fed from a common primary winding or from separateprimary windings as shown, the power in either case being derived fromthe sounce 11.

One load of the output line 16 is connected to a variable point on theresistor 20, whilst one end of the winding 17 is connected to a variablepoint on the resistor 21. One end of the winding 18 is connected to aslider movable along the resistor 22 whilst one end of the winding 19 isconnected to a dilference amplifier 23 which also receives an input fromthe other lead of the output line 16. The output from the amplifier 23drives a motor 24 which in turn drives the slider of resistor 22 asindicated by a dotted line in the drawing.

The arrangement is such that the motor 24 drives the slider until theinputs to the amplifier 23 from the load cells 8 and the windings 17,18, 1? are equal, at which point any convenient indicating device 25operated by the motor shaft indicates the load on the weighbridge.

The variable point on the resistor 20 is pre-set to allow for the weightof the plate, so that a zero indication will be obtained with no load onthe Weighbridge. The variable point on the resistor 21 can be adjustedto change the zero reading so that, for example, the Weighbridge readszero when an unladen lorry is on the weighbridge.

The arrangement described has the advantage that the three secondarywindings 17, 18, 19 act independently, so that adjustment of oneresistor does not necessitate adjustment of the other resistors.

A further disadvantage with weighbridges of the general type to whichthis invention relates is that there is often a phase difference betweenthe input voltage to the load cells and the voltage developed by theload cells which adversely affects the operation of the measuringapparatus. It has previously been the practice to over come thisdifiiculty by a lengthy process of trial and error 9 involving earthingvarious points in the electrical circuit through capacitors. In thepresent example, however, this difiiculty is overcome simply byconnecting a resistor 26 acros the amplifier and earching a pointintermediate the ends thereof, the precise point to be earthed beingdetermined by trial and error.

Having thus described my invention what I claim as new and desire tosecure by Letters Patent is:

1. A weighbridge including a plurality of load cells to which the loadto be measured is applied, in which the load cells are connected inparallel to a common output cable through independently variableresistors respectively, the resistances of the resistors being so chosenthat the output impedances of the load cells are equal.

2. A weighbridge as claimed in claim 1 in which the load cells areconnected in parallel to a common input cable through resistorsrespectively whereby the sensitivities of the load cells can be varied.

3. A weighbridge comprising in combination a weighing station and arecording station, a plurality of load cells at the weighing station towhich the load to be measured is applied, an input cable extendingbetween the recording and weighing stations for supplying AC. to theload cells, the input cable incorporating a transformer intermediate itsends, an output cable for conducting the output from the load cell orcells to indicating apparatus at the recording station, and a pluralityof independently variable resistors through which the load cellsrespectively are connected in parallel to the output cable, theresistances of said resistors being chosen so that the output impedancesof the load cells are equal.

4. A weighbrid ge as claimed in claim 3 including a plunality ofresistors through which the load cells respectively are connected inparallel to the secondary of said transformer whereby the sensitivitiesof the load cells can be varied.

5. A weighbridge as claimed in claim 3 in which the transformer ispositioned at the weighing station in close proximity to the load cells.

6. A weighbridge as claimed in claim 3 in which the input and outputcables are separated by such a distance that current flowing in theinput cable has substantially no effect on current flowing in the outputcable.

References Cited ly the Examiner UNITED STATES PATENTS 2,478,000 8/49Miller 32375 2,815,480 12/57 Ruge 323-75 3,046,411 7/62 Steiner 323-753,094,184 6/63 Mating 177--211 3,100,546 8/63 Cramwinckel 177211 LEOSMILOW, Primary Examiner.

1. A WEIGHTBRIDGE INCLUDING A PLURALITY OF LOAD CELLS TO WHICH THE LOADTO BE MEASURED IS APPLIED, IN WHICH THE LOAD CELLS ARE CONNECTED INPARALLEL TO A COMMON OUTPUT CABLE THROUGH INDEPENDENTLY VARIABLERESISTORS RESPECTIVELY, THE RESISTANCES OF THE RESISTORS BEING SOCHOOSEN THAT THE OUTPUT IMPEDANCES OF THE LOAD CELLS ARE EQUAL.